U.S. patent application number 14/387421 was filed with the patent office on 2015-02-19 for mr imaging using apt contrast enhancement and sampling at multple echo times.
The applicant listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Holger Eggers, Jochen Keupp.
Application Number | 20150051474 14/387421 |
Document ID | / |
Family ID | 45939206 |
Filed Date | 2015-02-19 |
United States Patent
Application |
20150051474 |
Kind Code |
A1 |
Eggers; Holger ; et
al. |
February 19, 2015 |
MR IMAGING USING APT CONTRAST ENHANCEMENT AND SAMPLING AT MULTPLE
ECHO TIMES
Abstract
The invention relates to a method of CEST or APT MR imaging of
at least a portion of a body (10) placed in a main magnetic field
B.sub.0 within the examination volume of a MR device. The method of
the invention comprises the following steps: .cndot.a) subjecting
the portion of the body (10) to a saturation RF pulse at a
saturation frequency offset; .cndot.b) subjecting the portion of
the body (10) to an imaging sequence comprising at least one
excitation/refocusing RF pulse and switched magnetic field
gradients, whereby MR signals are acquired from the portion of the
body (10) as spin echo signals; .cndot.c) repeating steps a) and b)
two or more times, wherein the saturation frequency offset and/or a
echo time shift in the imaging sequence are varied, such that a
different combination of saturation frequency offset and echo time
shift is applied in two or more of the repetitions; .cndot.d)
reconstructing a MR image and/or B.sub.0 field homogeneity
corrected APT/CEST images from the acquired MR signals. Moreover,
the invention relates to a MR device (1) for carrying out the
method of the invention and to a computer program to be run on a MR
device.
Inventors: |
Eggers; Holger; (Eindhoven,
NL) ; Keupp; Jochen; (Eindhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
EINDHOVEN |
|
NL |
|
|
Family ID: |
45939206 |
Appl. No.: |
14/387421 |
Filed: |
March 21, 2013 |
PCT Filed: |
March 21, 2013 |
PCT NO: |
PCT/IB2013/052244 |
371 Date: |
September 23, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61619601 |
Apr 3, 2012 |
|
|
|
Current U.S.
Class: |
600/410 |
Current CPC
Class: |
G01R 33/5605 20130101;
G01R 33/243 20130101; G01R 33/5617 20130101; G01R 33/4828 20130101;
A61B 5/055 20130101 |
Class at
Publication: |
600/410 |
International
Class: |
G01R 33/48 20060101
G01R033/48 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2012 |
EP |
12162970.3 |
Claims
1. A method of MR imaging of at least a portion of a body placed in
a main magnetic field B.sub.0 within the examination volume of a MR
device, the method comprising the following steps: a) subjecting
the portion of the body to a saturation RF pulse at a saturation
frequency offset; b) subjecting the portion of the body to an
imaging sequence comprising at least one excitation/refocusing RF
pulse and switched magnetic field gradients, whereby MR signals are
acquired from the portion of the body as spin echo signals; c)
repeating steps a) and b) two or more times, wherein the saturation
frequency offset and/or a echo time shift in the imaging sequence
are varied, such that a different combination of saturation
frequency offset and echo time shift is applied in two or more of
the repetitions; d) reconstructing a MR image from the acquired MR
signals.
2. Method of claim 1 wherein a number of offset-values for the
saturation frequency offset and a number of shift-values for the
echo-time shift are selected and for respective different selected
offset-value applied for a saturation RF pulse at a saturation
frequency offset a different shift value for the echo time shift is
applied in the imaging sequence.
3. Method of claim 2, wherein the applied offset-values and the
applied shift values effect a sparse sampling of the plane spanned
by offset values and shift values.
4. Method of claim 1, wherein contributions from fat spins and
water spins to the acquired MR signals are separated on the basis
of a single-point or multi-point Dixon technique.
5. Method of claim 1, wherein the spatial variation of B.sub.0
within the portion of the body is determined from the acquired MR
signals by means of a multi-point Dixon technique based on the MR
signal acquisitions with saturation frequency offsets that are
positive with respect to the resonance frequency of water
protons.
6. Method of claim 1, wherein the reconstruction of the MR image
includes deriving the spatial distribution of amide protons within
the portion of the body from an asymmetry analysis of the amplitude
of the acquired MR signals as a function of the saturation
frequency, which asymmetry analysis involves a saturation frequency
offset correction based on the determined spatial variation of
B.sub.0.
7. Method of claim 6, wherein the reconstruction of the MR image
includes deriving the spatial pH distribution within the portion of
the body from an asymmetry analysis of the amplitude of the
acquired MR signals as a function of the saturation frequency,
which asymmetry analysis involves a saturation frequency offset
correction based on the determined spatial variation of
B.sub.0.
8. Method of claim 1, wherein saturation RF pulses are applied in
different repetitions of steps a) and b) at positive and negative
saturation frequency offsets around the resonance frequency of
water protons.
9. Method of claim 1, wherein steps a) and b) are repeated two or
more times with the same saturation frequency offset and with a
different echo time shift in two or more of the repetitions.
10. Method of claim 1, wherein steps a) and b) are repeated two or
more times with a different saturation frequency offset and with a
different echo time shift in two or more of the repetitions.
11. MR device comprising: at least one main magnet coil for
generating a uniform, steady magnetic field within an examination
volume; a number of gradient coils for generating switched magnetic
field gradients in different spatial directions within the
examination volume; at least one RF coil for generating RF pulses
within the examination volume and/or for receiving MR signals from
a body of a patient positioned in the examination volume; a control
unit for controlling the temporal succession of RF pulses and
switched magnetic field gradients; and a reconstruction unit for
reconstructing a MR image from the received MR signals, wherein the
MR device is arranged to perform the following steps: a) subjecting
the portion of the body to a saturation RF pulse at a saturation
frequency offset; b) subjecting the portion of the body to an
imaging sequence comprising at least one excitation/refocusing RF
pulse and switched magnetic field gradients, whereby MR signals are
acquired from the portion of the body as spin echo signals; c)
repeating steps a) and b) two or more times, wherein the saturation
frequency offset and/or a echo time shift in the imaging sequence
are varied, such that a different combination of saturation
frequency offset and echo time shift is applied in two or more of
the repetitions; d) reconstructing an MR image as B.sub.0 field
homogeneity corrected APT/CEST images from the acquired MR
signals.
12. Computer program to be run on a MR device, which computer
program comprises instructions for: a) generating a saturation RF
pulse at a saturation frequency offset; b) generating an imaging
sequence comprising at least one excitation/refocusing RF pulse and
switched magnetic field gradients, whereby MR signals are acquired
from the portion of the body as spin echo signals; c) repeating
steps a) and b) two or more times, wherein the saturation frequency
offset and/or a echo time shift in the imaging sequence are varied,
such that a different combination of saturation frequency offset
and echo time shift is applied in two or more of the repetitions;
d) reconstructing an MR image as B.sub.0 field homogeneity
corrected APT/CEST images from the acquired MR signals.
Description
FIELD OF THE INVENTION
[0001] The invention relates to the field of magnetic resonance
(MR) imaging. It concerns a method of MR imaging of at least a
portion of a body placed in a main magnetic field B.sub.0 within
the examination volume of a MR device. The invention also relates
to a MR device and to a computer program for a MR device.
[0002] Image-forming MR methods which utilize the interaction
between magnetic fields and nuclear spins in order to form
two-dimensional or three-dimensional images are widely used
nowadays, notably in the field of medical diagnostics, because for
the imaging of soft tissue they are superior to other imaging
methods in many respects, do not require ionizing radiation and are
usually not invasive.
BACKGROUND OF THE INVENTION
[0003] According to the MR method in general, the body of the
patient to be examined is arranged in a strong, uniform magnetic
field B.sub.0 whose direction at the same time defines an axis
(normally the z-axis) of the co-ordinate system on which the
measurement is based. The magnetic field produces different energy
levels for the individual nuclear spins in dependence on the
magnetic field strength which can be excited (spin resonance) by
application of an electromagnetic alternating field (RF field) of
defined frequency (so-called Larmor frequency, or MR frequency).
From a macroscopic point of view, the distribution of the
individual nuclear spins produces an overall magnetization which
can be deflected out of the state of equilibrium by application of
an electromagnetic pulse of appropriate frequency (RF pulse) while
the magnetic field of the RF pulse extends perpendicular to the
z-axis, so that the magnetization performs a precession about the
z-axis. This motion of the magnetization describes a surface of a
cone whose angle of aperture is referred to as flip angle. The
magnitude of the flip angle is dependent on the strength and the
duration of the applied electromagnetic pulse. In the case of a
so-called 90.degree. pulse, the spins are deflected from the z axis
to the transverse plane (flip angle 90.degree.). The RF pulse is
radiated toward the body of the patient via a RF coil arrangement
of the MR device. The RF coil arrangement typically surrounds the
examination volume in which the body of the patient is placed.
[0004] After termination of the RF pulse, the magnetization relaxes
back to the original state of equilibrium, in which the
magnetization in the z direction is built up again with a first
time constant T.sub.1 (spin lattice or longitudinal relaxation
time), and the magnetization in the direction perpendicular to the
z direction relaxes with a second time constant T.sub.2 (spin-spin
or transverse relaxation time). The variation of the magnetization
can be detected by means of receiving RF coils which are arranged
and oriented within the examination volume of the MR device in such
a manner that the variation of the magnetization is measured in the
direction perpendicular to the z-axis. The decay of the transverse
magnetization is accompanied, after application of, for example, a
90.degree. pulse, by a transition of the nuclear spins (induced by
local magnetic field inhomogeneities) from an ordered state with
the same phase to a state in which all phase angles are uniformly
distributed (dephasing). The dephasing can be compensated by means
of a refocusing pulse (for example a 180.degree. pulse). This
produces an echo signal (spin echo) in the receiving coils.
Alternatively, the dephasing can be compensated by means of a
magnetic gradient pulse, producing an echo signal (gradient echo)
in the receiving coils. In order to realize spatial resolution in
the body, linear magnetic field gradients extending along the three
main axes are superposed on the uniform magnetic field, leading to
a linear spatial dependency of the spin resonance frequency. The
signal picked up in the receiving coils then contains components of
different frequencies which can be associated with different
locations in the body. The signal data obtained via the receiving
coils corresponds to the spatial frequency domain and are called
k-space data. The k-space data usually include multiple lines
acquired with different phase encoding. Each line is digitized by
collecting a number of samples. A set of k-space data is converted
to a MR image by means of Fourier transformation.
[0005] In some medical applications, the difference in MR signal
intensity from standard MR protocols, i.e. the contrast between
different tissues, might not be sufficient to obtain satisfactory
clinical information. In this case, contrast enhancing techniques
are applied. A particularly promising approach for contrast
enhancement and increase of MR detection sensitivity (by orders of
magnitude) is the known method based on `Chemical Exchange
Saturation Transfer` (CEST), as initially described by Balaban et
al. (see e.g. U.S. Pat. No. 6,962,769 B1) for the application to
exogenously administered contrast agents. According to the CEST
technique, the image contrast is obtained by altering the intensity
of the water proton signal in the presence of a contrast agent or
an endogenous molecule with a proton pool resonating at a different
frequency than the main water resonance. This is achieved by
selectively saturating the nuclear magnetization of the pool of
exchangeable protons which resonate at a frequency different from
the water proton resonance. Exchangeable protons can be provided by
exogenous CEST contrast agents (e.g. DIACEST, PARACEST or LIPOCEST
agents), but can also be found in biological tissue (i.e.,
endogenous amide protons in proteins and peptides, protons in
glucose or protons in metabolites like choline or creatinine) A
frequency-selective saturation RF pulse that is matched to the MR
frequency (chemical shift) of the exchangeable protons is used for
this purpose. The saturation of the MR signal of the exchangeable
protons is subsequently transferred to the MR signal of nearby
water protons within the body of the examined patient by chemical
exchange with the water protons, thereby decreasing the water
proton MR signal. The selective saturation at the MR frequency of
the exchangeable protons thus gives rise to a negative contrast in
a water proton based MR image. Amide proton transfer (APT) MR
imaging, which is a CEST technique based on endogenous exchangeable
protons, allows highly sensitive and specific detection of
pathological processes on a molecular level, like increased protein
concentrations in malignant tumor tissue. The APT signal is also
sensitively reporting on locally altered pH levels--because the
exchange rate is pH dependent--which can e.g. be used to
characterize acidosis in ischemic stroke. APT/CEST MR imaging has
several advantages over conventional MR contrasts. APT/CEST MR
imaging allows highly specific detection and differentiation of
endogenous contrasts, which is much more sensitive then e.g.
spectroscopic MR/NMR techniques. This high sensitivity (SNR
efficiency) can be used to obtain molecular contrast information at
a resolution comparable to typical MR imaging applications in
clinically acceptable examination times. Furthermore, CEST
contrasts allow for multiplexing by using a single molecules or a
mixture of molecules bearing exchangeable protons that can be
addressed separately in a multi-frequency CEST MR examination. This
is of particular interest for molecular imaging, where multiple
biomarkers may be associated with several unique CEST frequencies.
Moreover, the MR contrast in APT/CEST MR imaging can be turned on
and off at will by means of the frequency selective preparation RF
pulse. Adjustable contrast enhancement is highly advantageous in
many applications, for example when the selective uptake of the
contrast agent in diseased tissue in the examined body is slow, or
for increasing the specificity of detection in areas with highly
structured basic MR contrast.
[0006] In conventional APT and CEST MR imaging, the effect of the
saturation transfer of exchangeable protons to water is identified
by an asymmetry analysis of the amplitude of the acquired MR
signals as a function of the saturation frequency. This asymmetry
analysis is performed with respect to the MR frequency of water
protons, which, for convenience, is assigned to a saturation
frequency offset of 0 ppm. The measurement of the amplitude of the
acquired MR signals as a function of the saturation frequency
offset and the asymmetry analysis are inherently very sensitive to
any inhomogeneity of the main magnetic field B.sub.0. This is
because a small shift of the center frequency (e.g. a saturation
frequency offset of 0.1 ppm relative to the chemical shift of
water) easily causes a variation of more than 10% in the asymmetry
data. This variation results in large artifacts in the finally
reconstructed APT/CEST MR images.
[0007] It has been shown (e.g. Zhou et al., Magnetic Resonance in
Medicine, 60, 842-849, 2008) that the B.sub.0 inhomogeneity can be
corrected in APT/CEST imaging on a voxel-by-voxel basis through
re-centering of the asymmetry data on the basis of a separately
acquired B.sub.0 map. However, an additional B.sub.0 mapping scan
is required in this known approach. This results in an extended
overall imaging time. Several other known methods to correct for
B.sub.0 inhomogeneity require additional overall scan time to
obtain the necessary B.sub.0 field inhomogeneity information (e.g.
WASSR). Moreover, the separately performed measurement of to obtain
the B.sub.0 inhomogeneity information is potentially inaccurate or
inconsistent, for example because of patient motion, shimming or
frequency drift of the used MR device between the field mapping and
the actual APT/CEST acquisition. Consequently, the B.sub.0 map has
to be acquired in close temporal proximity to the APT/CEST scan and
potentially needs to be repeated, for example in order to ensure
sufficient precision in case of multiple APT/CEST scans within one
examination. Thus, the known technique may be severely limited for
clinical applications with respect to scan time efficiency and
precision.
[0008] Another issue in APT and CEST MR imaging is that a robust
elimination of signal contributions from fat spins, e.g. by fat
saturation RF pulses, is often difficult in the presence of B.sub.0
inhomogeneity. However, residual fat signal contributions result in
a strongly biased asymmetry of the amplitude of the acquired MR
signals as a function of the saturation frequency offset near the
chemical shift of fat protons at -3.4 ppm relative to the MR
frequency of water protons. This is of particular concern in
applications in which MR images of organs with significant fat
content are to be acquired, such as the liver or the breast.
[0009] The ISMRM 2010 abstract `CEST-Dixon MRI for sensitive and
accurate measurement of amide proton transfer in human 3T` by J.
Keupp and H. Eggers discloses a multi-echo T1-weighted gradient
echo sequence to acquire APT/CEST MRI. This known approach also
employs an iterative Dixon technique to map local field
inhomogeneities based on a multi-echo gradient echo approach. This
approach provides a B.sub.0 field map acquired during the actual
APT/CEST acquisition and thus solves some of the above mentioned
issues related to additional scan time and workflow/timing for the
field mapping as well as the precision of the field
characterization,
SUMMARY OF THE INVENTION
[0010] From the foregoing it is readily appreciated that there is a
need for an improved MR imaging technique. It is consequently an
object of the invention to provide a MR imaging method and a MR
device which enable high-quality and high contrast-to-noise MR
imaging using APT/CEST with efficient and precise intrinsic B.sub.0
determination and possible robust elimination of adverse effects
due to fat signal contributions.
[0011] In accordance with the invention, a method of MR imaging of
at least a portion of a body placed in a main magnetic field
B.sub.0 within the examination volume of a MR device is disclosed.
The method of the invention comprises the following steps:
a) subjecting the portion of the body to a saturation RF pulse at a
saturation frequency offset; b) subjecting the portion of the body
to an imaging sequence comprising at least one excitation RF pulse
and switched magnetic field gradients, whereby MR signals are
acquired from the portion of the body as spin echo signals; c)
repeating steps a) and b) two or more times, wherein the saturation
frequency offset and/or a echo time shift in the imaging sequence
are varied, such that a different combination of saturation
frequency offset and echo time shift is applied in two or more of
the repetitions; d) reconstructing a MR image from the acquired MR
signals. In accordance with the invention, the portion of the body
is subjected to saturation RF pulses each having a saturation
frequency offset relative to the MR frequency of water protons.
Corresponding to conventional APT/CEST MR imaging, saturation RF
pulses are irradiated at different saturation frequency offsets
(e.g. near +/-3.5 ppm) around the MR frequency of water protons (0
ppm). After each saturation step, MR signals are acquired by means
of a spin echo-type sequence. Preferably a fast spin echo (FSE) or
turbo spin echo (TSE) sequence is applied because of the high SNR
efficiency provided by this sequence type. Also combined spin echo
and gradient echo sequences like the GRASE sequence could be
employed. As the compensation of the dephasing is typically less
complete in gradient echo base MR pulse sequences due to local
variations of the magnetic field (T2* decay), a higher
signal-to-noise-ratio (SNR) is achieved in spin echo based MR
techniques.
[0012] According to the invention, the combinations of saturation
frequency offset values and echo time shift values is kept limited.
In effect the plane spanned by the offset values and shift values
is sparsely sampled by the interation of the imaging sequence at
the selected combinations of offset values and shift values. As the
number of interations at respective combinations of offset-values
and shift-values is limited, the total acquisition time of the
APT/CEST acquisition can be limited. In the optimal case, according
to the invention, the extra time for B.sub.0 field mapping is fully
avoided, while the overall acquisition time for a conventional
APT/CEST acquisition (requiring B.sub.0 information in addition) is
not significantly increased. As an example, an efficient 2D APT
sampling scheme uses 7 different saturation frequency offsets (e.g.
-4, -3.5 , -3, +3, +3.5, +4.5 ppm and one image without or with far
detuned saturation). The steps of saturation and signal acquisition
are repeated, wherein the saturation frequency offset and/or the
echo time shift in the imaging sequence are varied. This can be
achieved, for example, by varying the timing of the RF refocusing
pulse(s), thereby shifting the refocusing of the nuclear
magnetization, and/or by varying the timing of the signal
acquisition window and associated magnetic field gradients. An
essential feature of the invention is that different and selected
combinations of saturation frequency offset and echo time shift are
applied in different repetitions. A subset of specific combinations
are selected which can be efficiently used to obtain APT/CEST
images with intrinsic magnetic field homogeneity correction.
Finally, MR images is reconstructed from the acquired MR signals,
which may be quantitative APT/CEST images or APT/CEST weighted
images.
[0013] The known technique is strictly limited to gradient echo
type MR sequences, which excludes the use of highly
contrast-to-noise ratio efficient spin echo type acquisitions. A
spin echo based Dixon technique is not scan time efficient for
APT/CEST, because it requires multiple full image acquisitions with
different echo shifts for one or even for all saturation frequency
offsets. Thus, in a conventional spin echo Dixon approach, the
APT/CEST overall acquisition time would be significantly increased
to obtain the field inhomogeneity information, similar to the
techniques using separate B.sub.0 mapping. For a conventional
3-point spin echo Dixon reconstruction of the field inhomogeneity
(to provide one B.sub.0 map, no fat separation across the
z-spectrum), at least one of the offsets has to be fully acquired 3
times. This would require at least two additional full images and a
scan time increase of 30%. Separate B.sub.0 mapping (e.g. dual-echo
gradient echo based) needs about the same or even more extra scan
time. B.sub.0 map information needs to be acquired with sufficient
SNR, thus, typically about 1 minute of acquisition time is required
2D (and more for 3D).
[0014] Since MR signals are acquired as spin echo signals at
different echo time shifts, the spatial variation of B.sub.0 within
the portion of the body can be determined from the acquired MR
signals by means of a multi-point Dixon technique. According to the
per se known Dixon technique, the spectral difference between fat
and water spins is made use of for the purpose of separating MR
signals emanating from water containing tissue and MR signals
emanating from fat tissue. In spin echo Dixon imaging, multiple
acquisitions of k-space are repeated with different echo time
shifts. The simplest Dixon technique, a two-point Dixon technique,
acquires two complete k-space data sets, wherein the fat
magnetization in the second acquisition shows a phase difference
(e.g. 180.degree.=out phase) relative to the water magnetization,
and a different phase difference (e.g. 0.degree.=in phase) in the
first acquisition. In the case of out phase and in phase images,
separate and distinct water and fat images can be obtained by
simple addition or subtraction of the complex MR signal data. In
general, a B.sub.0 field map, a water image and a fat image are
obtained by means of a Dixon technique, which may include an
iterative reconstruction approach. Hence, also the spatial
variation of B.sub.0 within the portion of the body can be
determined from the MR signals acquired in accordance with the
invention by means of the single- or multi-point spin echo Dixon
technique. The method of the invention thus permits the application
of Dixon methods for both B.sub.0 mapping as well as water/fat
separation simultaneously in the context of spin echo MRI. The
method of the invention integrates spin echo Dixon methods into
APT/CEST MR imaging in an efficient manner.
[0015] The reconstruction of the MR image according to the
invention may include deriving the spatial distribution of amide
protons within the portion of the body from an asymmetry analysis
or other z-spectral analysis technique based on the amplitude of
the acquired MR signals as a function of the saturation frequency
offset, wherein the z-spectral analysis involves a saturation
frequency offset correction based on the spatial variation of
B.sub.0 determined by means of the applied Dixon method. The
approach of the invention thus enables correcting for B.sub.0
inhomogeneity in APT/CEST MR imaging by integration of spin echo
Dixon methods.
[0016] Moreover, the reconstruction of the MR image according to
the invention may include deriving the spatial pH distribution
within the portion of the body from the asymmetry analysis or other
z-spectral analysis technique on the basis of the amplitude of the
acquired MR signals as a function of the saturation frequency.
Again, the z-spectral analysis may involve a saturation frequency
offset correction based on the determined spatial variation of
B.sub.0.
[0017] According to the invention, the saturation RF pulses are
applied in different repetitions of steps a) and b) at positive and
negative saturation frequency offsets around the resonance
frequency of water protons. As in conventional APT/CEST MR imaging,
different saturation frequency offsets (e.g. near +/-3.5 ppm)
around the MR frequency of water protons are applied. Steps a) and
b) may be repeated two or more times with the same saturation
frequency offset but simultaneously applying a different echo time
shift in each repetition. This could by implemented such that the
acquisition with any saturation frequency offset is repeated two or
three times, each with a different echo time shift. Alternatively,
steps a) and b) are repeated two or more times with a different
saturation frequency offset and with a different echo time shift in
two or more of the repetitions. This means that both the saturation
frequency offset and the echo time shift are simultaneously varied
in the repetitions. The latter scheme is preferably applied for
saturation frequency offsets that are positive with respect to the
resonance frequency of water protons. For positive saturation
frequency offsets, the amplitude of the MR signals of water protons
can be expected to vary only slightly between the individual
repetitions of steps a) and b) due to different extents of direct
saturation of water protons and due to the relevant saturation
transfer effects e.g. near +3.5 ppm (APT), while the MR signal
amplitude of fat protons is expected to stay constant. In order to
assure the condition of minor amplitude variations of the order of
<10% among the repetitions with different frequency offsets and
echo shifts, the saturation frequency offsets of at least two
repetitions need to be placed in close frequency proximity (e.g.
0.5 ppm apart for APT). In contrast, the contribution of fat
protons to the overall MR signal amplitude may be modulated
substantially at negative saturation frequency offsets in the
proximity of the saturation frequency corresponding to the chemical
shift-induced frequency of fat protons (-3.4 ppm). Therefore, the
spin echo Dixon type B.sub.0 mapping, according to the invention,
is preferably based on the MR signal acquisitions with positive
saturation frequency offsets. The obtained B.sub.0 map can
subsequently be employed for water-fat separation at all saturation
frequency offsets, e.g. by means of a single-point or multi-point
Dixon technique.
[0018] The method of the invention described thus far can be
carried out by means of a MR device including at least one main
magnet coil for generating a uniform steady magnetic field within
an examination volume, a number of gradient coils for generating
switched magnetic field gradients in different spatial directions
within the examination volume, at least one RF coil for generating
RF pulses within the examination volume and/or for receiving MR
signals from a body of a patient positioned in the examination
volume, a control unit for controlling the temporal succession of
RF pulses and switched magnetic field gradients, and a
reconstruction unit for reconstruction of a MR image from the
received MR signals. The method of the invention is preferably
implemented by a corresponding programming of the control unit
and/or the reconstruction unit of the MR device.
[0019] The method of the invention can be advantageously carried
out in most MR devices in clinical use at present. To this end it
is merely necessary to utilize a computer program by which the MR
device is controlled such that it performs the above-explained
method steps of the invention. The computer program may be present
either on a data carrier or be present in a data network so as to
be downloaded for installation in the control unit of the MR
device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The enclosed drawings disclose preferred embodiments of the
present invention. It should be understood, however, that the
drawings are designed for the purpose of illustration only and not
as a definition of the limits of the invention. In the
drawings:
[0021] FIG. 1 shows a MR device according to the invention;
[0022] FIG. 2 shows a diagram illustrating the scheme of saturation
frequency offsets used for APT MR imaging according to the
invention,
[0023] FIG. 3 shows a diagram illustrating a first embodiment of
the APT acquisition scheme according to the invention;
[0024] FIG. 4 shows a diagram illustrating a second embodiment of
the APT acquisition scheme according to the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0025] With reference to FIG. 1, a MR device 1 is shown. The device
comprises superconducting or resistive main magnet coils 2 such
that a substantially uniform, temporally constant main magnetic
field B.sub.0 is created along a z-axis through an examination
volume. The device further comprises a set of (1.sup.st, 2.sup.nd,
and--where applicable--3.sup.rd order) shimming coils 2', wherein
the current flow through the individual shimming coils of the set
2' is controllable for the purpose of minimizing B.sub.0 deviations
within the examination volume.
[0026] A magnetic resonance generation and manipulation system
applies a series of RF pulses and switched magnetic field gradients
to invert or excite nuclear magnetic spins, induce magnetic
resonance, refocus magnetic resonance, manipulate magnetic
resonance, spatially and otherwise encode the magnetic resonance,
saturate spins, and the like to perform MR imaging.
[0027] More specifically, a gradient pulse amplifier 3 applies
current pulses to selected ones of whole-body gradient coils 4, 5
and 6 along x, y and z-axes of the examination volume. A digital RF
frequency transmitter 7 transmits RF pulses or pulse packets, via a
send-/receive switch 8, to a body RF coil 9 or to a set of local
array RF coils 11, 12, 13, to transmit RF pulses into the
examination volume. A typical MR imaging sequence is composed of a
packet of RF pulse segments which, together with any applied
magnetic field gradients, achieve a selected manipulation of
nuclear magnetic resonance. The RF pulses are used to saturate,
excite resonance, invert magnetization, refocus resonance, or
manipulate resonance and select a portion of a body 10 positioned
in the examination volume. The MR signals are also picked up by the
body RF coil 9.
[0028] For generation of MR images of limited regions of the body
10 by means of parallel imaging, a set of local array RF coils 11,
12, 13 are placed contiguous to the region selected for imaging.
The array coils 11, 12, 13 can be used to receive MR signals
induced by body-coil RF transmissions.
[0029] The resultant MR signals are picked up by the body RF coil 9
and/or by the array RF coils 11, 12, 13 and demodulated by a
receiver 14 preferably including a preamplifier (not shown). The
receiver 14 is connected to the RF coils 9, 11, 12 and 13 via
send-/receive switch 8.
[0030] A host computer 15 controls the shimming coils 2' as well as
the gradient pulse amplifier 3 and the transmitter 7 to generate
any of a plurality of MR imaging sequences, such as echo planar
imaging (EPI), echo volume imaging, gradient and spin echo imaging,
fast spin echo imaging, and the like. For the selected sequence,
the receiver 14 receives a single or a plurality of MR data lines
in rapid succession following each RF excitation pulse. A data
acquisition system 16 performs analog-to-digital conversion of the
received signals and converts each MR data line to a digital format
suitable for further processing. In modern MR devices the data
acquisition system 16 is a separate computer which is specialized
in the acquisition of raw image data.
[0031] Ultimately, the digital raw image data are reconstructed
into an image representation by a reconstruction processor 17 which
applies a Fourier transform or other appropriate reconstruction
algorithms, such as SENSE or GRAPPA. The MR image may represent a
planar slice through the patient, an array of parallel planar
slices, a three-dimensional volume, or the like. The image is then
stored in an image memory where it may be accessed for converting
slices, projections, or other portions of the image representation
into appropriate format for visualization, for example via a video
monitor 18 which provides a man-readable display of the resultant
MR image.
[0032] In accordance with the invention, the portion of the body 10
is subjected to saturation RF pulses at different saturation
frequency offsets prior to acquisition of MR signals by means of a
spin echo sequence, which is preferably a fast spin echo (FSE) or
turbo spin echo (TSE) sequence or a related pulse sequence like
GRASE (a combined spin echo and gradient echo sequence). The
saturation RF pulses are irradiated via the body RF coil 9 and/or
via the array RF coils 11, 12, 13, wherein the saturation frequency
offset relative to the MR frequency of water protons is set by
appropriate control of the transmitter 7 via the host computer 15.
As shown in FIG. 2, different saturation frequency offsets are
applied around +/-3.5 ppm around the MR frequency of water protons
(0 ppm). The different saturation frequency offsets are indicated
by black arrows in FIG. 2. A further reference acquisition may be
performed "off-resonant", i.e. with a very large frequency offset
which leaves the MR signal amplitude of water protons unaffected or
with the RF saturation power switched off, which is useful for
signal normalization (quantification of the z-spectral asymmetry).
This is indicated by the leftmost black arrow in FIG. 2.
[0033] According to the invention, MR signal acquisition steps are
repeated several times, wherein the saturation frequency offset and
the echo time shifts in the spin echo sequence are varied, such
that a different combination of saturation frequency offset and
echo time shift is applied in two or more of the repetitions.
Finally, an APT/CEST MR image is reconstructed from the acquired MR
signals. This means that the reconstruction of the MR image
includes deriving the spatial distribution of amide protons within
the portion of the body 10 from an asymmetry analysis or a similar
z-spectral analysis technique base on the amplitude of the acquired
MR signals as a function of the saturation frequency offset. This
z-spectral analysis, which is conventionally applied in APT/CEST MR
imaging, is very sensitive to any inhomogeneity of the main
magnetic field B.sub.0. This is taken into account by the method of
the invention by determination of the spatial variation of B.sub.0
from the acquired MR signals by means of a multi-point Dixon
technique. The determined spatial variation of B.sub.0 is then used
for a corresponding saturation frequency offset correction in the
asymmetry analysis or other z-spectral analysis technique.
[0034] For a determination of the spatial variation of B.sub.0, two
specific strategies may be applied in accordance with the
invention. These strategies are illustrated in the diagrams of
FIGS. 3 and 4.
[0035] The saturation steps are indicated in FIGS. 3 and 4 by
SAT-3, SAT-2, SAT-1, SAT0, SAT+1, SAT+2 and SAT+3. Therein, SAT-1,
SAT-2 and SAT-3 correspond to negative saturation frequency
offsets, while SAT+1, SAT+2 and SAT+3 correspond to positive
saturation frequency offsets. SAT0 corresponds to a reference
measurement, in which an off-resonant frequency offset is applied,
as mentioned above. ACQ1, ACQ2, ACQ3 and ACQ4 indicate MR signal
acquisition steps using different echo time shifts (TE.sub.1,
TE.sub.2, TE.sub.3, TE.sub.4), respectively.
[0036] In the embodiment shown in FIG. 3, an acquisition with any
saturation frequency offset SAT-3, SAT-2, SAT-1, SAT0, SAT+1, SAT+2
and SAT+3 is repeated three times, each with a different echo time
shift, indicated by ACQ1, ACQ2, and ACQ3. This results in an
overall number of 21 repetitions. B.sub.0 mapping is preferably
performed separately for each saturation frequency offset.
[0037] In the further embodiments shown FIG. 4, the acquisitions
with different saturation frequency offsets SAT-3, SAT-2, SAT-1,
SAT+1, SAT+2, SAT+3 are performed only once, but with different
echo time shifts, indicated by ACQ1, ACQ2, ACQ3 and ACQ4 (echo
times TE.sub.1, TE.sub.2, TE.sub.3, TE.sub.4). ACQ0 indicates an
acquisition without echo time shift (echo time TE.sub.0). A
multi-point (iterative) Dixon technique is applied to derive the
B.sub.0 map from these acquisitions combining the data from
different saturation frequency offsets, according to the invention.
In FIG. 4a three different echo time shifts (indicated by ACQ1,
ACQ2, ACQ3) are applied with the saturation frequency offsets
SAT+1, SAT+2 and SAT+3. The B.sub.0 map is derived from these
acquisitions. No echo time shift is applied in the acquisitions
with SAT-3, SAT-2, SAT-1 and SAT0. The B.sub.0 map is applied for
correction in these acquisitions. In FIG. 4b the different echo
time shifts are also applied with SAT-3, SAT-2, SAT-1. No echo time
shift is applied for SAT0. In FIG. 4c three different echo time
shifts (that are well-suited for 3-point Dixon B.sub.0 mapping) are
applied with saturation frequency offsets SAT+1, SAT+2, SAT+3,
while a single echo time shift that is well-suited for water/fat
separation (indicated by ACQ4) is applied with saturation frequency
offsets SAT0, SAT-1, SAT-2 and SAT-3. In FIG. 4d no echo time shift
is applied for SAT-3, SAT-2, SAT-1 and SAT+1, SAT+2, SAT+3, while
three different echo time shifts are applied with SAT-0 (for
B.sub.0 mapping).
[0038] For positive saturation frequency offsets which are placed
in close spectral proximity of the chemical shift of the
exchangeable proton pool in question (e.g. +3.5 ppm for APT), the
MR signal amplitude of water protons is expected to vary slightly
(<10%) between the individual acquisitions due to different
extents of direct saturation of water protons and due to the
relevant saturation transfer effects, as mentioned above. The
resulting signal variation may be addressed in different ways for
the purpose of B.sub.0 mapping. One option is to simply ignore this
small signal variation. This option can be used in practice, in
particular in combination with specifically positioned saturation
frequency offsets, but it may potentially result in a somewhat
reduced precision of the determined B.sub.0 map. Another option is
to minimize the influence of the signal variations by choosing
appropriate echo time shifts, where the Dixon-based B.sub.0
determination is most robust against signal variations. A further
option is to apply an appropriate mathematical model of the
acquired composite complex MR signals and to derive the B.sub.0
from the resulting model parameters. Different strategies for MR
signal modeling in Dixon imaging exist, which can be applied in
accordance with the invention, and which are per se known in the
art.
[0039] In an embodiment of the invention, the composite complex
signal S acquired with SAT+1, SAT+2, SAT+3 may be modeled by:
S.sub.+1=(W.sub.1+c.sub.1F)P.DELTA.P*
S.sub.+2=(W.sub.2+c.sub.2F)P
S.sub.+3=(W.sub.3+c.sub.3F)P.DELTA.P
or, by using a linear approximation, as:
S.sub.+1=(W-.DELTA.W+c.sub.1F)P.DELTA.P*
S.sub.+2=(W+c.sub.2F)P
S.sub.+3=(W+.DELTA.W+c.sub.3F)P.DELTA.P,
wherein W denotes the water signal contribution, F denotes the fat
signal contribution, P and .DELTA.P denote phase errors, and c
denotes coefficients that describe the amplitude and phase of a
unit fat signal at the respective echo time shift. W, F, P, and
.DELTA.P are considered as unknowns, while S and c are considered
as knowns. In the first case (without approximation), the number of
knowns (real and imaginary components of S) and the number of
unknowns (real W.sub.1-W.sub.3, real F, phase of P and .DELTA.P)
are both equal to six. In the second case (with approximation), the
number of knowns exceeds the number of unknowns by one. The
acquisition with saturation frequency offset SAT0 may be included
as fourth equation, again with a different W and the same F.
B.sub.0 can be derive on a voxel-by-voxel basis from the resulting
model parameters.
[0040] The spatial variation of B.sub.0 can be assumed not to
change between the individual MR signal acquisition steps to
acquire the different saturation frequency offsets for APT/CEST
MRI. Accordingly, once the spatial variation of B.sub.0 has been
determined in the afore-described manner, the obtained B.sub.0 map
can be used for suppression of signal contributions from fat spins.
A Dixon method can be applied to perform a water/fat separation
after demodulation of B.sub.0-induced phase errors. The echo time
values can be optimized to maximize the signal-to-noise ratio in
the resulting water MR images, for instance by choosing echo time
shifts at which signal contributions from water and fat spins are
in quadrature, i.e. 90.degree. out of phase. If other echo time
values are preferred for B.sub.0 mapping than are favorable for
Dixon water/fat separation, some acquisitions with appropriate
saturation frequency offsets may be repeated with correspondingly
chosen echo time values.
[0041] For positive saturation frequency offsets, one of the
schemes illustrated in FIG. 4 for obtaining the B.sub.0 map can
also be employed to suppress signal contributions from fat spins.
For the acquisitions with negative saturation frequency offsets
near the chemical shift of fat protons, the extent of saturation of
fat protons imposed by the saturation RF pulses can be modeled on
the basis of an appropriate mathematical model, taking the RF pulse
parameters (for example shape, bandwidth) and the spectrum of the
fat protons (for example number of peaks, resonance frequencies,
resonance areas, line widths) into account.
[0042] In an exemplary embodiment, the composite signal S acquired
with SAT-1, SAT-2, SAT-3 may be modeled as:
S.sub.-1=(W.sub.1+c.sub.1d.sub.1F)P.DELTA.P*
S.sub.-2=(W.sub.2+c.sub.2d.sub.2F)P
S.sub.--3=(W.sub.3+c.sub.3d.sub.3F)P.DELTA.P
or, using a linear approximation, as:
S.sub.-1=(W-.DELTA.W+c.sub.1d.sub.1F)P.DELTA.P*
S.sub.-2=(W+c.sub.2d.sub.2F)P
S.sub.-3=(W+.DELTA.W+c.sub.3d.sub.3F)P.DELTA.P,
wherein d denotes coefficients that describe the relative extent of
fat suppression. For the acquisitions with both, positive and
negative saturation frequency offsets, F may be considered as
unknown, or F may be considered as known from the water/fat
separation in the acquisition with off-resonant saturation
SAT0.
[0043] After water/fat separation, an APT/CEST MR image at the
desired saturation offset frequency (e.g. +3.5 ppm for APT) can be
reconstructed by means of the above-mentioned asymmetry analysis or
other z-spectral analysis technique based on the voxel-wise
amplitude of the water MR images as a function of the saturation
frequency offset. Therein, the asymmetry/z-spectral analysis
involves a saturation frequency offset correction based on the
determined spatial variation of B.sub.0., e.g. by means of a
voxel-by-voxel Lagrange interpolation of the images taken at
different saturation frequency offsets.
* * * * *